The present invention relates to formation testing of a producing formation in an oil or gas well. Formation testing helps determine the potential productivity of a subsurface formation intersected by a well bore. The testing procedure requires the opening of a section of the well bore adjacent the formation to atmospheric or reduced pressure. A "testing string" comprising a string of drill pipe having incorporated therein a tester valve and one or more packers is lowered into the well bore, which may be cased or open hole, with the tester valve closed to prevent entry of well bore fluids into the string. Two packers may be employed if it is necessary to isolate the formation to be tested from the well bore below it. At the desired level, the packer or packers are set to isolate the formation to be tested, and the formation is then exposed to reduced pressure in the empty pipe string by opening the tester valve. The initial ability of the formation to produce fluid is thereby determined, and the tester valve is subsequently closed after a predetermined time period to test the rate of pressure buildup in the formation. This sequence may be repeated several times. At the end of the test, the tester valve is closed, pressure across the packer or packers is equalized, after which they are unset, and the testing string removed from the well. Formation pressures and in some instances other parameters, are recorded by one or more combination measuring and recording devices included in the testing string below the tester valve.
Several prior art methods of measuring downhole parameters are known in the art. The most common utilizes one or more combination measuring and recording devices placed in housings incorporated in the testing string. These devices are activated prior to the string being run into the well, and their proper operation or accuracy cannnot, as a result, be determined until the testing string is pulled from the well bore. Another more recently developed device of the prior art employs a wireline with an actuator sub at the bottom thereof to latch into a measuring device incorporated in the string, whereby a real time readout of the formation parameters measured is obtained at the surface. However, there is again the disadvantage of not being able to replace a faulty or inoperative measuring device without pulling the entire testing string, as well as the possibility that the wireline connection or the wireline itself may short out during the test, a common occurrence due to the hostile environment in the well bore and the long duration of the tests which often extend to several days. A third type of device employed to measure downhole parameters is a so-called "bomb hanger," such as is available from Otis Engineering Corporation of Dallas, Tex., whereby an instrument may be run into the well at the end of a wireline on the bomb hanger and locked into a collar recess on the interior of a pipe string, and retrieved in the same manner. This device, however, does not provide an absolutely positive indication that the instrument is locked in where desired, and also creates a significant flow obstruction when placed in the pipe string. In addition, the relatively large diameter of the bomb hanger precludes it from being run below any reduced diameter portion in a pipe string, such as in a testing string below a ball type tester valve, in close proximity to the formation.